Various patents and papers have been issued and published for developing synthetic polymers for water-based drilling fluids that offer desired rheology control as well as fluid-loss control for high-temperature applications up to 260° C. (500° F.) (if lasts 16 hours at 204° C. (400° F.). These polymers are generally linear or lightly crosslinked (less than 1 mol % crosslinker) and need to be used with clay to achieve desired viscosity and fluid-loss control. The use of clay, however, can cause severe formation damage because it plugs the pores of the reservoir formation and is very difficult to be cleaned up. Without clay, unfortunately, these polymers alone cannot provide desired viscosity and fluid-loss control.
US Patent Publication No. 20110168393 discloses a crosslinked homopolymer of polyacrylamide as high-temperature fluid-loss additive. The crosslinkers are either diethylene glycol dimethacrylate, N,N′-ethylenebisacrylamide, or polyethylene glycol diacrylate. Two issues remain for this polymer. First, since the polymer is a homopolymer of acrylamide, it is not compatible with the sized calcium carbonate (bridging agent) after aging because the polymer hydrolyzes to polyacrylate. As a result, the calcium carbonate bridging agent and the polymer precipitate out of the mud formulation after aging. Second, the crosslinkers can also be hydrolyzed after aging, resulting loss of viscosity and fluid loss control.